Human megakaryocyte-associated tyrosine kinase (MATK)-related gene variant associated with lung cancers

ABSTRACT

The invention relates to the nucleic acid of a novel human MATK-related variant (MATKV) and the polypeptides encoded thereby. The invention also provides a process for producing the polypeptide encoded by MATKV. The invention further provides the uses of the nuclei acid of MATKV and the polypeptide encoded thereby in diagnosing the disease associated with the deficiency of human MATK gene, in particular lung cancers, preferably small cell lung cancers.

This is a Divisional of U.S. application Ser. No. 10/103,380 filed on Mar. 21, 2002 now U.S. Pat. No. 6,908,755 issued Jun. 21, 2005.

FIELD OF THE INVENTION

The invention relates to the nucleic acid of a novel megakaryocyte-associated tyrosine kinase (MATK)-related gene variant, the polypeptide encoded thereby, the preparation process thereof, and the uses of the same in diagnosing diseases associated with the deficiency of human MATK gene, in particular, lung cancers.

BACKGROUND OF THE INVENTION

Lung cancer is one of the major causers of cancer-related deaths in the world. There are two primary types of lung cancers: small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC) (Carney, (1992a) Curr. Opin. Oncol. 4:292–8). Small cell lung cancer accounts for approximately 25% of lung cancer and spreads aggressively (Smyth et al. (1986) Q J Med. 61: 969–76; Carney, (1992b) Lancet 339: 843–6). Non-small cell lung cancer represents the majority (about 75%) of lung cancer and is further divided into three main subtypes: squamous cell carcinoma, adenocarcinoma, and large cell carcinoma (Ihde and Minna, (1991) Cancer 15: 105–54). In recent years, much progress has been made toward understanding the molecular and cellular biology of lung cancers. Many important contributions have been made by the identification of several key genetic factors associated with lung cancers. However, the treatments of lung cancers still mainly depend on surgery, chemotherapy and radiotherapy. This is because the molecular mechanisms underlying the pathogenesis of lung cancers remain largely unclear.

A recent hypothesis suggests that lung cancer is caused by genetic mutations of at least 10 to 20 genes (Sethi, (1997) BMJ. 314: 652–655). One of the future strategies for the prevention and treatment of SCLC will be focused on the elucidation of the genes associated with protooncogene, in particular, the c-kit gene. This suggestion is based on the evidence that the c-kit gene was found to be expressed preferentially in SCLC (Sekido et al. (1991) Cancer Res 51:2416–9), which indicates an important role of the c-kit gene in the tumorigenic process of SCLC. One of the genes associated with the c-kit is the megakaryocyte-associated tyrosine kinase (MATK) (Jhun et al. (1995) J Biol Chem 270: 9661–6; Price et al. (1997) J Biol Chem 272:5915–20). MATK, a protein tyrosine kinase containing the Src homology 2 and 3 (SH2 and SH3) domains, was also termed Csk-homologous kinase (CHK) based on its high sequence similarity to the Csk tyrosine kinase. The functional roles of MATK have been shown to be involved in cell proliferation and differentiation, and chromosome dynamics (Avraham et al. (1995) J Biol Chem 270:1833–42; Zrihan-Licht et al. (1998) J Biol Chem 273:4065–72; Yamashita et al. (1999) J Biol Chem 274:15059–65; Yamaguchi et al. (2001) J Cell Sci 114:1631–41). The genetic localization analysis has assigned MATK to chromosome 19p13.3 (Avraham et al. (1995) J Biol Chem 270:1833–42). Several studies have shown that the abnormality of chromosome 19 is associated with lung tumor susceptibility (Johansson et al. (1995) Cancer Genet Cytogenet 80:85–6; Dang et al. (2000) J Natl Cancer Inst 92:1355–7; Wang et al. (2000) Clin Cancer Res 6:2988–93; Sobottka et al. (2000) J Neurooncol 49:187–95). Interestingly, the gene variants of a chromosome 19 gene, C-CAM1, were shown to be involved in lung tumorigenesis (Wang et al. (2000) Clin Cancer Res 6:2988–93) raising the possibility that the gene variants of MATK may also be involved in the tumorigenic process of SCLC. Therefore, it is believed that the discovery of gene variants of MATK may serve as important targets for diagnostic markers of SCLC.

SUMMARY OF THE INVENTION

The present invention provides a MATK-related gene variant (MATKV) present in human SCLC tissues. The nucleic acid of MATKV and the polypeptide encoded thereby can be used for the diagnosis of diseases associated with the deficiency of human MATK gene, in particular lung cancers, preferably SCLC.

The invention further provides an expression vector and host cell for expressing MATKV.

The invention further provides a method for producing the polypeptide encoded by MATKV.

The invention further provides an antibody specifically binding to the polypeptide encoded by MATKV.

The invention also provides methods for diagnosing diseases associated with the deficiency of human MATK gene, in particular, lung cancers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A to 1D show the nucleotide sequence of MATKV (SEQ ID NO:1) and the corresponding amino acid sequence thereof (SEQ ID NO:2).

FIG. 2A to 2I show the nucleotide sequence alignment between human MATK gene and MATKV.

FIG. 3A to 3C show the amino acid sequence alignment between human MATK protein and the polypeptide encoded by MATKV.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, all technical and scientific terms used have the same meanings as commonly understood by persons skilled in the art.

The term “antibody,” as used herein, denotes intact molecules (a polypeptide or group of polypeptides) as well as fragments thereof, such as Fab, R(ab′)₂, and Fv fragments, which are capable of binding the epitopic determinutesant. Antibodies are produced by specialized B cells after stimulation by an antigen. Structurally, antibody consists of four subunits including two heavy chains and two light chains. The internal surface shape and charge distribution of the antibody binding domain are complementary to the features of an antigen. Thus, antibody can specifically act against the antigen in an immune response.

The term “base pair (bp),” as used herein, denotes nucleotides composed of a purine on one strand of DNA which can be hydrogen bonded to a pyrimidine on the other strand. Thymine (or uracil) and adenine residues are linked by two hydrogen bonds. Cytosine and guanine residues are linked by three hydrogen bonds.

The term “Basic Local Alignment Search Tool (BLAST; Altschul et al., (1997) Nucleic Acids Res. 25: 3389–3402),” as used herein, denotes programs for evaluation of homologies between a query sequence (amino or nucleic acid) and a test sequence as described by Altschul et al. (Nucleic Acids Res. 25: 3389–3402, 1997). Specific BLAST programs are described as follows:

(1) BLASTN compares a nucleotide query sequence against a nucleotide sequence database;

(2) BLASTP compares an amino acid query sequence against a protein sequence database;

(3) BLASTX compares the six-frame conceptual translation products of a query nucleotide sequence against a protein sequence database;

(4) TBLASTN compares a query protein sequence against a nucleotide sequence database translated in all six reading frames; and

(5) TBLASTX compares the six-frame translations of a nucleotide query sequence against the six-frame translations of a nucleotide sequence database.

The term “cDNA,” as used herein, denotes nucleic acids that synthesized from a MRNA template using reverse transcriptase.

The term “cDNA library,” as used herein, denotes a library composed of complementary DNAs which are reverse-transcribed from mRNAs.

The term “complement,” as used herein, denotes a polynucleotide sequence capable of forming base pairing with another polynucleotide sequence. For example, the sequence 5′-ATGGACTTACT-3′ binds to the complementary sequence 5′-AGTAAGTCCAT-3′.

The term “deletion,” as used herein, denotes a removal of a portion of one or more amino acid residues/nucleotides from a gene.

The term “expressed sequence tags (ESTs),” as used herein, denotes short (200 to 500 base pairs) nucleotide sequence that derives from either 5′ or 3′ end of a cDNA.

The term “expression vector,” as used herein, denotes nucleic acid constructs which contain a cloning site for introducing the DNA into vector, one or more selectable markers for selecting vectors containing the DNA, an origin of replication for replicating the vector whenever the host cell divides, a terminator sequence, a polyadenylation signal, and a suitable control sequence which can effectively express the DNA in a suitable host. The suitable control sequence may include promoter, enhancer and other regulatory sequences necessary for directing polymerases to transcribe the DNA.

The term “host cell,” as used herein, denotes a cell which is used to receive, maintain, and allow the reproduction of an expression vector comprising DNA. Host cells are transformed or transfected with suitable vectors constructed using recombinant DNA methods. The recombinant DNA introduced with the vector is replicated whenever the cell divides.

The term “insertion” or “addition,” as used herein, denotes the addition of a portion of one or more amino acid residues/nucleotides to a gene.

The term “in silico,” as used herein, denotes a process of using computational methods (e.g., BLAST) to analyze DNA sequences.

The term “polymerase chain reaction (PCR),” as used herein, denotes a method which increases the copy number of a nucleic acid sequence using a DNA polymerase and a set of primers (about 20 bp oligonucleotides complementary to each strand of DNA) under suitable conditions (successive rounds of primer annealing, strand elongation, and dissociation).

The term “protein” or “polypeptide,” as used herein, denotes a sequence of amino acids in a specific order that can be encoded by a gene or by a recombinant DNA. It can also be chemically synthesized.

The term “nucleic acid sequence” or “polynucleotide,” as used herein, denotes a sequence of nucleotide (guanine, cytosine, thymine or adenine) in a specific order that can be a natural or synthesized fragment of DNA or RNA. It may be single-stranded or double-stranded.

The term “reverse transcriptase-polymerase chain reaction (RT-PCR),” as used herein, denotes a process which transcribes MRNA to complementary DNA strand using reverse transcriptase followed by polymerase chain reaction to amplify the specific fragment of DNA sequences.

The term “transformation,” as used herein, denotes a process describing the uptake, incorporation, and expression of exogenous DNA by prokaryotic host cells.

The term “transfection,” as used herein, a process describing the uptake, incorporation, and expression of exogenous DNA by eukaryotic host cells.

The term “variant,” as used herein, denotes a fragment of sequence (nucleotide or amino acid) inserted or deleted by one or more nucleotides/amino acids.

In the first aspect, the present invention provides the polypeptide encoded by a novel human MATK-related gene variant (MATKV) and the fragments thereof, as well as the nucleic acid of MATKV.

According to the present invention, human MATK cDNA sequence was used to query the human lung EST databases (a normal lung, a large cell lung cancer, and a small cell lung cancer) using BLAST program to search for MATK-related gene variants. One human cDNA partial sequences (i.e., EST) showing similarity to MATK was identified from ESTs deposited in the SCLC database. The cDNA clone, named MATKV (MATK gene variant), was then isolated from the SCLC cDNA library and sequenced. FIGS. 1A to 1D show the nucleotide sequence (SEQ ID NO: 1) of MATKV and its corresponding amino acid sequence (SEQ ID NO: 2).

The full-length of the MATKV cDNA is a 2065 bp clone containing a 1659 bp open reading frame (ORF) extending from 263 bp to 1921 bp, which corresponds to an encoded protein of 553 amino acid residues with a predicted molecular mass of 61.2 kDa. The sequence around the initiation ATG codon of MATKV (located at nucleotide 263 to 265 bp) was matched to the Kozak consensus sequence (A/GCCATGG) (Kozak, (1987) Nucleic Acids Res. 15: 8125–48; Kozak, (1991) J Cell Biol. 115: 887–903.). To determine the variation in sequence of MATKV cDNA clone, an alignment of MATK nucleotide/amino acid sequence with MATKV was performed (FIGS. 2 and 3). One major genetic insertion was found in the aligned sequences, which shows that MATKV contains a 78 bp insertion in the sequence of human MATK gene from nucleotides 844 to 845. This 78 bp insertion is positioned from nucleotides 845 to 922 of the nucleotide sequence of MATKV, which encode the amino acids 195 to 220 of SEQ ID NO: 2.

In the present invention, a search of ESTs deposited in dbEST (Boguski et al. (1993) Nat Genet. 4: 332–3) at National Center for Biotechnology Information (NCBI) was performed to determine the tissue distribution of MATKV in silico. The result of in silico Northern analysis showed that three ESTs (GenBank accession number BE795492; BE791829; BE791472) were found to confirm the addition of 78 bp nucleotide sequence inserted on the MATKV. These ESTs were generated from the SCLC CDNA library, which suggests that the addition of 78 bp nucleotide fragment located between nucleotides 845 to 922 of MATKV may serve as a useful marker for diagnosing SCLC. Therefore, any nucleotide fragments comprising nucleotides 845 to 922 of MATKV may be used as probes for determining the presence of MATKV under high stringency conditions. An alternative approach is that any set of primers for amplifying the fragment containing nucleotides 845 to 922 of MATKV may be used for determining the presence of human MATK-related gene variant.

Scanning the MATKV sequence against the profile entries in PROSITE (ScanProsite) indicated that MATKV protein contains two N-glycosylation sites (83–86aa and 199–202aa), eight protein kinase C phosphorylation sites (9–11aa, 26–28aa, 39–41aa, 44–46aa, 149–151aa, 371–373aa, 409–411aa, and 427–429aa), seven casein kinase II phosphorylation sites (18–21aa, 55–58aa, 76–79aa, 94–97aa, 128–131aa, 179–182aa, and 244–247aa), eleven N-myristoylation sites (5–10aa, 15–20aa, 98–103aa, 198–203aa, 227–232aa, 241–246aa, 264–269aa, 282–287aa, 336–341aa, 398–403aa, and 406–411aa), one protein kinases ATP-binding region signature (267–288aa), and one tyrosine protein kinases specific active-site signature (374–386aa). A search of the MATKV sequence against the protein profile databases (ProfileScan) indicated that MATKV protein contains one src homology 3 (SH3) domain (58–110aa), one Src homology 2 (SH2) domain (122–194aa), one proline-rich region (470–540aa), and one protein kinase domain (261–540aa).

According to the present invention, the polypeptide encoded by MATKV and the fragments thereof may be produced through genetic engineering techniques. For instance, they may be produced by using appropriate host cells which have been transformed with recombinant DNAs that code for the desired polypeptides or fragments thereof. The nucleotide sequence of MATKV or the fragments thereof is inserted into an appropriate expression vector, i.e., a vector which contains the necessary elements for the transcription and translation of the inserted coding sequence in a suitable host. The nucleotide sequence is inserted into the vector in a manner such that it will be expressed under appropriate conditions (e.g., in proper orientation and correct reading frame and with appropriate expression sequences, including an RNA polymerase binding sequence and a ribosomal binding sequence).

Any method that is known to those skilled in the art may be used to construct expression vectors containing the sequence of MATKV and appropriate transcriptional/translational control elements. These methods may include in vitro recombinant DNA and synthetic techniques, and in vivo genetic recombinant techniques. (See, e.g., Sambrook, J. Cold Spring Harbor Press, Plainview N.Y., Ch. 4, 8, and 16–17; Ausubel, R. M. et al. (1995) Current protocols in Molecular Biology, John Wiley & Sons, New York N.Y., ch. 9, 13, and 16.)

A variety of expression vector/host systems may be utilized to express MATKV. These include, but not limited to, microorganisms such as bacteria transformed with recombinant bacteriophage, plasmid, or cosmid DNA expression vector; yeast transformed with yeast expression vector; insect cell systems infected with virus (e.g., baculovirus); plant cell system transformed with viral expression vector (e.g., cauliflower mosaic virus, CaMV, or tobacco mosaic virus, TMV); or animal cell system infected with virus (e.g., vaccina virus, adenovirus, etc.). Preferably, the host cell is a bacterium, and more preferably, the bacterium is E. coli.

Alternatively, the polypeptide encoded by MATKV or the fragments thereof may be synthesized using chemical methods. For example, peptide synthesis can be performed using various solid-phase techniques (Roberge, J. Y. et al. (1995) Science 269: 202 to 204). Automated synthesis may be achieved using the ABI 431A peptide synthesizer (Perkin-Elmer).

According to the present invention, the fragments of the nucleic acid of MATKV and the polypeptide encoded thereby are used as primers or probes and immunogens, respectively. Preferably, the purified fragments are used. The fragments may be produced by enzyme digestion, chemical cleavage of isolated or purified polypeptide or nucleotide sequences, or chemical synthesis, and then may be isolated or purified. Such isolated or purified fragments of the polypeptides and nucleotide sequences can be used directed as immunogens and primers or probes, respectively.

The present invention further provides the antibodies which specifically bind to one or more out-surface epitopes of the polypeptide encoded by MATKV.

According to the present invention, immunization of mammals with immunogens described herein, preferably humans, rabbits, rats, mice, sheep, goats, cows, or horses, is performed by following the procedures well known to those skilled in the art, for the purpose of obtaining antisera containing polyclonal antibodies or hybridoma lines secreting monoclonal antibodies.

Monoclonal antibodies can be prepared by standard techniques, given the teachings contained herein. Such techniques are disclosed, for example, in U.S. Pat. Nos. 4,271,145 and 4,196,265. Briefly, an animal is immunized with the immunogen. Hybridomas are prepared by fusing spleen cells from the immunized animal with myeloma cells. The fusion products are screened for those producing antibodies that bind to the immunogen. The positive hybridoma clones are isolated, and the monoclonal antibodies are recovered from those clones.

Immunization regimens for production of both polyclonal and monoclonal antibodies are well-known in the art. The immunogen may be injected by any of a number of routes, including subcutaneous, intravenous, intraperitoneal, intradermal, intramuscular, mucosal, or a combination thereof. The immunogen may be injected in soluble form, aggregate form, attached to a physical carrier, or mixed with an adjuvant, using methods and materials well-known in the art. The antisera and antibodies may be purified using column chromatography methods well known to those skilled in the art.

According to the present invention, antibody fragments which contain specific binding sites for the polypeptides or fragments thereof may also be generated. For example, such fragments include, but are not limited to, F(ab′)₂ fragments produced by pepsin digestion of the antibody molecule and Fab fragments generated by reducing the disulfide bridges of the F(ab′)₂ fragments.,

Many gene variants have been found to be associated with diseases (Stallings-Mann et al., (1996) Proc Natl Acad Sci USA 93: 12394–9; Liu et al., (1997) Nat Genet 16:328–9; Siffert et al., (1998) Nat Genet 18: 45 to 8; Lukas et al., (2001) Cancer Res 61: 3212 to 9). Since MATKV clone was isolated from SCLC cDNA library and its expression in SCLC was confirmed by in silico Northern analysis, it suggests that MATKV may serve as marker for the diagnosis of diseases associated with the deficiency of human MATK gene, in particular lung cancers, e.g. human SCLC. Thus, the expression level of MATKV relative to MATK may be a useful indicator for screening of patients suspected of having such diseases, and the index of relative expression level (mRNA or protein) may confer an increased susceptibility to the same.

Accordingly, the subject invention further provides methods for diagnosing the diseases associated with the deficiency of human MATK gene in a mammal, in particular, lung cancers, preferably the SCLC.

The method for diagnosing the diseases associated with the deficiency of human MATK gene may be performed by detecting the nucleotide sequence of the MATKV of the invention, which comprises the steps of: (1) extracting the total RNA of cells obtained from the mammal; (2) amplifying the RNA by reverse transcriptase-polymerase chain. reaction (RT-PCR) with a set of primers to obtain a cDNA comprising the fragments comprising nucleotides 845 to 922 of SEQ ID NO: 1; and (3) detecting whether the cDNA is obtained. If necessary, the amount of the obtained CDNA sample may be determined.

In this embodiment, one of the primers may be designed to have a sequence comprising the nucleotides of SEQ ID NO: 1 containing nucleotides 845 to 922, and the other may be designed to have a sequence complementary to the nucleotides of SEQ ID NO: 1 at any other locations downstream of nucleotide 922. Alternatively, one of the primers may be designed to have a sequence complementary to the nucleotides of SEQ ID NO: 1 containing nucleotides 845 to 922, and the other may be designed to have a sequence comprising the nucleotides of SEQ ID NO: 1 at any other locations upstream of nucleotide 845. In this case, only MATKV will be amplified.

Alternatively, one of the primers may be designed to have a sequence comprising the nucleotides of SEQ ID NO: 1 upstream of nucleotide 844 and the other may be designed to have a sequence complementary to the nucleotides of SEQ ID NO: 1 downstream of nucleotide 923. Alternatively, one of the primers may be designed to have a sequence complementary to the nucleotides of SEQ ID NO: 1 upstream of nucleotide 844 and the other may be designed to have a sequence comprising the nucleotides of SEQ ID NO: 1 downstream of nucleotide 923. In this case, both MATK and MATKV will be amplified. The length of the PCR fragment from MATKV will be 78 bp longer than that from MATK.

Preferably, the primer of the invention contains 15 to 30 nucleotides.

Total RNA may be isolated from patient samples by using TRIZOL reagents (Life Technology). Tissue samples (e.g., biopsy samples) are powdered under liquid nitrogen before homogenization. RNA purity and integrity are assessed by absorbance at 260/280 nm and by agarose gel electrophoresis. The set of primers designed to amplify the expected sizes of specific PCR fragments of MATKV can be used. PCR fragments are analyzed on a 1% agarose gel using five microliters (10%) of the amplified products. To determine the expression levels for each gene variants, the intensity of the PCR products may be determined by using the Molecular Analyst program (version 1.4.1; Bio-Rad).

The RT-PCR experiment may be performed according to the manufacturer instructions (Boehringer Mannheim). A 50 μl reaction mixture containing 2 μl total RNA (0.1 μg/μl), 1 μl each primer (20 pM), 1 μl each dNTP (10 mM), 2.5 μl DTT solution (100 mM), 10 μl 5X RT-PCR buffer, 1 μl enzyme mixture, and 28.5 μl sterile distilled water may be subjected to the conditions such as reverse transcription at 60° C. for 30 minutes followed by 35 cycles of denaturation at 94° C. for 2 minutes, annealing at 60° C. for 2 minutes, and extension at 68° C. for 2 minutes. The RT-PCR analysis may be repeated twice to ensure reproducibility, for a total of three independent experiments.

Another embodiment of the method for diagnosing the diseases associated with the deficiency of human MATK gene of the invention may be performed by detecting the nucleotide sequence of MATKV, which comprises the steps of: (1) extracting total RNA from a sample obtained from the mammal; (2) amplifying the RNA by reverse transcriptase-polymerase chain reaction (RT-PCR) to obtain a cDNA sample; (3) bringing the cDNA sample into contact with the nucleic acid of SEQ ID NO: 1 or the fragments thereof; and (4) detecting whether the cDNA sample hybridizes with the nucleic acid. If necessary, the amount of the hybridized sample may be determined.

The expression of gene variants can also be analyzed using Northern blot hybridization approach. Specific fragments comprising nucleotides 845 to 922 of the MATKV may be amplified by polymerase chain reaction (PCR) using primer set designed for RT-PCR. The amplified PCR fragment may be labeled and serve as a probe to hybridize the membranes containing total RNAs extracted from the samples under the conditions of 55° C. in a suitable hybridization solution for 3 hr. Blots may be washed twice in 2×SSC, 0.1% SDS at room temperature for 15 minutes each, followed by two washes in 0.1×SSC and 0.1% SDS at 65° C. for 20 minutes each. After these washes, blot may be rinsed briefly in suitable washing buffer and incubated in blocking solution for 30 minutes, and then incubated in suitable antibody solution for 30 minutes. Blots may be washed in washing buffer for 30 minutes and equilibrated in suitable detection buffer before detecting the signals. Alternatively, the presence of gene variants (cDNAs or PCR) can be detected using microarray approach. The cDNAs or PCR products corresponding to the nucleotide sequences of the invention may be immobilized on a suitable substrate such as a glass slide. Hybridization can be preformed using the labeled mRNAs extracted from samples. After hybridization, nonhybridized mRNAs are removed. The relative abundance of each labeled transcript, hybridizing to a cDNA/PCR product immobilized on the microarray, can be determined by analyzing the scanned images.

According to the invention, the method for diagnosing the diseases associated with the deficiency of human MATK gene may also be performed by detecting the polypeptides encoded by the MATKV of the invention. For instance, the polypeptide in protein samples obtained from the mammal may be determined by, but not limited to, the immunoassay wherein the antibodies specifically binding to the polypeptides of the invention is contacted with the sample, and the antibody-polypeptide complex is detected. If necessary, the amount of antibody-polypeptide complex can be determined.

The polypeptides encoded by MATKV may be expressed in prokaryotic cells by using suitable prokaryotic expression vectors. The CDNA fragments of MATKV may be PCR-amplified using primer set with restriction enzyme digestion sites incorporated in the 5′ and 3′ ends, respectively. The PCR products can then be enzyme digested, purified, and inserted into the corresponding sites of prokaryotic expression vector in-frame to generate recombinant plasmids. Sequence fidelity of this recombinant DNA can be verified by sequencing. The prokaryotic recombinant plasmids may be transformed into host cells (e.g., E. coli BL21 (DE3)). Recombinant protein synthesis may be stimulated by the addition of 0.4 mM isopropylthiogalactoside (IPTG) for 3 h. The bacterially-expressed proteins may be purified.

The polypeptides encoded by MATKV may be expressed in animal cells by using eukaryotic expression vectors. Cells may be maintained in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum (FBS; Gibco BRL) at 37° C. in a humidified 5% CO₂ atmosphere. Before transfection, the nucleotide sequence of each of the gene variant may be amplified with PCR primers containing restriction enzyme digestion sites and ligated into the corresponding sites of eukaryotic expression vector in-frame. Sequence fidelity of this recombinant DNA can be verified by sequencing. The cells may be plated in 12-well plates one day before transfection at a density of 5×10⁴ cells per well. Transfections may be carried out using Lipofectaminutese Plus transfection reagent according to the manufacturer's instructions (Gibco BRL). Three hours following transfection, medium containing the complexes may be replaced with fresh medium. Forty-eight hours after incubation, the cells may be scraped into lysis buffer (0.1 M Tris HCl, pH 8.0, 0.1% Triton X-100) for purification of expressed proteins. After these proteins are purified, monoclonal antibodies against these purified proteins may be generated using hybridoma technique according to the conventional methods (de StGroth and Scheidegger, (1980) J Immunol Methods 35:1–21; Cote et al. (1983) Proc Natl Acad Sci USA 80: 2026–30; and Kozbor et al. (1985) J Immunol Methods 81:31–42).

According to the invention, the presence of the polypeptides encoded by MATKV in samples of normal lung and lung cancers may be determined by, but not limited to, Western blot analysis. Proteins extracted from samples may be separated by SDS-PAGE and transferred to suitable membranes such as polyvinylidene difluoride (PVDF) in transfer buffer (25 mM Tris-HCI, pH 8.3, 192 mM glycine, 20% methanol) with a Trans-Blot apparatus for 1 h at 100 V (e.g., Bio-Rad). The proteins can be immunoblotted with specific antibodies. For example, membrane blotted with extracted proteins may be blocked with suitable buffers such as 3% solution of BSA or 3% solution of nonfat milk powder in TBST buffer (10 mM Tris-HCI, pH 8.0, 150 mM NaCl, 0.1% Tween 20) and incubated with monoclonal antibody specific to the polypeptides encoded by these gene variants. Unbound antibody is removed by washing with TBST for 5×1 minutes. Bound antibody may be detected using commercial ECL Western blotting detecting reagents.

The following examples are provided for illustration, but not for limiting the invention.

EXAMPLES Analysis of Human Lung EST Databases

Expressed sequence tags (ESTs) generated from the large-scale PCR-based sequencing of the 5′-end of human lung (normal, SCLC, and large cell lung cancer) cDNA clones were compiled and served as EST databases. Sequence comparisons against the nonredundant nucleotide and protein databases were performed using BLASTN and BLASTX programs (Altschul et al., (1997) Nucleic Acids Res. 25: 3389–3402; Gish and States, (1993) Nat Genet 3:266–272), at the NCBI with a significance cutoff of p<10⁻¹⁰. ESTs representing putative MATKV gene were identified during the course of EST generation.

Isolation of cDNA Clones

One cDNA clone exhibiting EST sequence similar to the MATK gene was isolated from the SCLC cDNA library and named MATKV. The inserts of these clones were subsequently excised in vivo from the λZAP Express vector using the ExAssist/XLOLR helper phage system (Stratagene). Phagemid particles were excised by coinfecting XL1-BLUE MRF' cells with ExAssist helper phage. The excised pBluescript phagemids were used to infect E. coli XLOLR cells, which lack the amber suppressor necessary for ExAssist phage replication. Infected XLOLR cells were selected using kanamycin resistance. Resultant colonies contained the double stranded phagemid vector with the cloned cDNA insert. A single colony was grown overnight in LB-kanamycin, and DNA was purified using a Qiagen plasmid purification kit.

Full Length Nucleotide Sequencing and Database Comparisons

Phagemid DNA was sequenced using the Epicentre#SE9101LC SequiTherm EXCEL™II DNA Sequencing Kit for 4200S-2 Global NEW IR² DNA sequencing system (LI-COR). Using the primer-walking approach, full-length sequence was determined. Nucleotide and protein searches were performed using BLAST against the non-redundant database of NCBI.

In Silico Tissue Distribution (Northern) Analysis

The coding sequence for each cDNA clones was searched against the dbEST sequence database (Boguski et al., (1993) Nat Genet. 4: 332–3) using the BLAST algorithm at the NCBI website. ESTs derived from each tissue were used as a source of information for transcript tissue expression analysis. Tissue distribution for each isolated cDNA clone was determined by ESTs matching to that particular sequence variants (insertions or deletions) with a significance cutoff of p<10⁻¹⁰.

References

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1. An isolated nucleic acid encoding the polypeptide comprising the amino acid sequence of SEQ ID NO:
 2. 2. The isolated nucleic acid of claim 1, which comprises the nucleotide sequence of SEQ ID NO:
 1. 3. An expression vector comprising the nucleic acid of claim
 1. 4. A host cell transformed with the expression vector of claim
 3. 